The present invention relates to an improved method and apparatus for cementing an acetabular cup into an acetabulum.
The idea of implanting a total prosthetic device as replacement for a damaged or diseased hip joint is not a new one, having been repeated, in one form or another, numerous times since the close of the last century. It was not, however, until more recently that the total hip movement began the upward spiral of concentrated interest known today. The ability to use self curing cement such as methyl merthacrylate in orthopedics enabled the total hip concept to become a workable reality. The cement sets up within minutes at surgery and overcomes the problem of transferring weight bearing forces from the artificial device to the natural bony structures of the pelvis and femur. Even with this improvement, the present techniques of cementing an acetabular cup to the acetabulum has many problems which must be overcome.
Many acetabuli which need cup arthroplasty are irregular in shape with a somewhat flattened dome of the acetabulum. This is true because the dome in most instances has been bearing most of the weight and is the area of the greatest wear. It should be understood that while the acetabuli differ in shape corresponding to the size of the individual, the amount of water, and the like, generally speaking, an acetabulum has a deep portion forming a spherical area with a dome which is the weight bearing portion of the acetabulum and a shallow portion forming a spherical zone of larger transverse diameter than the deeper spherical area with subchondral cortical bone and some cartilage. Of course, the outer edge of the acetabulum, which is the outer edge of the spherical zone, will be irregular by virtue of the body's natural design.
One of the greater problems of cement fixation of the cup arthroplasty has been getting good fixation at the cement/bone interface. Several efforts have been made to improve this cement/bone interface. One method has used plastic spacers between the bony acetabulum and the plastic cup to allow an acceptable amount of cement to rest between the acetabulum and the cup.
A second procedure has been to surgically place drill holes in the bony acetabulum and then fill the acetabulum (and the drilled holes) with cement of a doughy consistancy. After the doughy cement has been inserted in the acetabulum, an acetabular cup is inserted into the acetabulum and manual pressure is applied to this cup to force the cement between the trabeculae of the bone and into the drill holes. Usually, three drill holes are made in the acetabulum bone so that when such holes are filled with cement and the cement hardens, the cement has greater resistance to motion. However, such drill holes create an irregular and weakened dome of the spherical zone and creates irregular pressure areas on the bony acetabulum which may lead to failure of the bone structure in those areas. Further, the doughy cement at times does not adequately penetrate the bony structure of the acetabulum because the cement is not sufficiently liquid. Also, the cement may not uniformly cover and adhere to the acetabular cup.
Still another method of attempting to improve degenerated acetabuli has been the insertion of cup arthroplasty made of plastic material such as polyethylene, sometimes a metal shell with a plastic lining and more recently a plastic inner shell with an external metal shell covered externally with the irregular porous coating. In other cases, the use of ceramic cup arthroplasty has been used. Another procedure presently advocated is the use of porous coated cup arthroplasty which is secured by placement of the porous coating next to the reamed acetabulum or the use of methyl merthacrylate to cement the cup arthroplasty into place.
Still another procedure is to insert in the acetabulum a metal cup which is free to move. This method provided minimal or no additional support to the acetabular strength or durability. Further, with the use of the porous coated arthroplasty fixation by the body's reaction, repair does not always result in solid fixation.
In some cases, an injection of liquid methyl merthacrylate is injected into the holes drilled in the wall of the acetabulum to add strength to the medullary and to add greater resistance to the slipping of the hardened methyl merthacrylate.
Other surgeons employ an impacting procedure to force the methyl merthacrylate into the acetabulum. After a sufficient amount of the doughy methyl merthacrylate has been placed in the acetabulum, a plunger which circumscribes the rim of the acetabulum is applied to the acetabulum and manual pressure is applied to the impacter which applies a force to the cement. The impacter is then removed and only atmospheric pressure remains. The cup is then pushed into the dough like cement and held in place manually with a rigid instrument until the cement has hardened.
Another involves an acetabular cup having large screw threads on the outer periphery and the cup is actually threaded into the cup shaped acetabulum. Because of the irregular shape of the acetabulum, the threads may not contact the bone structure evenly along the threads. The porous coating and screw thread methods, in some instances, use supplemental bone graft between the acetabulum and the prosthesis.
All of the former methods of cementing the cup arthroplasty produce an undesirable number of loosenings of the cement and/or cup. Such loosenings are often accompanied by further degeneration or fracture of the bone which is in approximation to the cement.
Further, in order to prepare the acetabulum to receive the cup arthroplasty, the acetabulum is reamed with a device which produces a socket or cavity with a surface which is essentially spherical but frequently eliptical in contour. In reaming such socket or cavity, usually a large portion of the cortical like subchrondral bone is removed thus weakening the area further in addition to the bone weakening caused by the holes drilled in the wall of the acetabulum. Thus, only the medullary bone is left to react in a repairative manner.
Where porous coated acetabular cups are used, some surgeons allege that the bone grows into the irregular surface of the porous coating. Others believe that microfractures are created at the surface of the reamed acetabulum thus creating minute pieces of bone which act as bone grafts and that as these grafts heal, an osteoid structure is developed and conforms to the irregular surface of the porous coating.
Several problems develop with this type of hip reconstruction. The body's repair of the reamed acetabulum is necessarily time consuming of six weeks or more. Further, it may be accompanied by fibrous union rather than bony union. The development of a fibrous union weakens the strength of the bony acetabulum. The large amount of fibrous union delays the development of bony union and may even result in non-union of the bone and a loosening of the porous cup arthroplasty from the adjacent bone.
The proposed invention overcomes the disadvantages of the prior art by attaching the acetabular cup in spaced relationship to the acetabulum and forcing a flowable cement into the space between them under pressure so as to force the flowable cement partially into the medullary bone of the acetabulum.
In addition, the acetabulum is prepared by using a first sized reamer to remove the cartilage from the deepest portion of the acetabulum to form a spherical area in the weight bearing portion of the acetabulum and, where possible, by using a second, larger diameter reamer to remove the cartilage from the shallow portion of the acetabulum to form another spherical zone with a greater diameter and preserving at least a portion of the subchondral cortical like bone. Thus, uniformly curved surfaces are formed including a weight bearing surface which is a spherical area. These uniformly curved surfaces are stronger structurally than curved surfaces composed of areas with multiple and varied radii.
In addition, at least one annular groove is formed at least in the spherical zone of the acetabulum about the inner surface thereof to provide added fixation of the cement to the bone. Also, other annular grooves can be formed in the spherical zone and/or the spherical area of the acetabulum. Further, annular projections are formed on the external or outer surface of the acetabular cup prosthesis. These grooves in the reamed acetabulum and the projections on the external surface of the acetabulum cup are separated by a space and when liquid or flowable cement is forced into that space under pressure, the grooves and projections are filled and covered with cement which, when it hardens, produces a greater resistance to motion or loosening since a geometric circle structurally distributes the stress throughout the circle and thereby produces a relatively larger area of resistance than multiple small structural areas.
Thus, it is an object of the present invention to prepare an acetabulum for receiving an acetabulum cup by reaming the cartilage from the deepest portion of the acetabulum to form a spherical area in the weight bearing portion of the acetabulum.
It is a further object of the present invention, where the acetabuli so allows, to use a second, larger diameter reamer to remove the cartilage from the shallow portion of the acetabulum and form a larger spherical zone while preserving at least a major portion of the subchondral cortical like bone of the spherical area and the spherical zone of the acetabulum.
It is yet another object of the present invention to provide better cement fixation between said arthroplasty cup and the acetabulum by forming at least one annular groove in at least the spherical zone of said acetabular wall as well as forming annular projections about the periphery of the arthroplasty cup so that when cement sets within the annular grooves in the acetabulum and the annular grooves formed by the annular projections on the outer periphery of the cup, added fixation of the cement to the bone and the cup is provided.
It is still another object of the present invention to position the acetabular cup in spaced relationship to the acetabulum and inject a flowable cement under pressure into the space between the cup and the acetabulum to thereby force some of the flowable cement into the annular groove and into the medullary bone thereby forming a ring of cement with cortical like bone and a ring of cement in the madullary bone thereby forming a better cement fixation between the cup and the bone.
It is also an object of the present invention to provide an acetabular retaining ring to hold the acetabular cup in the required spaced relationship from the acetabulum while the flowable cement sets. Also, the retaining ring, when cemented in place, allows for later pressurizing of cement in the medullary bone and between the bone and cup arthroplasty.
It is yet another object of the present invention to provide a pliable sleeve having an annular projection about the periphery thereof which mates with the annular groove on the inner surface of the acetabulum to cover the groove and prevent cement from entering it while the retaining ring is being cemented to the acetabulum.
It is still another object of the present invention to provide an adjustment ring for placement between the acetabulum cup and the retainer ring for enabling the cup to be properly positioned for a particular acetabulum.